Method for the transfer of a gas of high purity

ABSTRACT

Method for transferring very pure ultimate use gas from a cylinder to a user apparatus through a tube fitted with valves and other distributing and control elements, the tube being fitted, more particularly stated, for regulating flow and pressure. The tubing, valves and other elements are subjected, before the transferring, and before assembly as a flow line, to chemical etching by aqueous solutions of volatile noncontaminating acids, then to decontamination by extensive sweeping by a very pure hot gas to volatilize the acids. The ultimate use gas is subsequently transferred, after assembly and additional sweeping by a pure hot gas, through the decontaminated tubing, valves and other elements, to the ultimate user apparatus, as for example in the transferring of dopantcontaining gas in the manufacturing of semiconductors by epitaxy or diffusion.

United States Patent Sifre et a1.

[ Apr. 29, 1975 METHOD FOR THE TRANSFER OF A GAS OF HIGH PURITY [75]Inventors: Gaston Sifre, Orsay; Rene Barandon, Morangis, both of France[73] Assignee: Societe Generale de Constructions Electriques etMecaniques (Alsthom), Paris, France [22] Filed: May 25, 1972 [21] Appl.No.: 256,765

[30] Foreign Application Priority Data May 27, 1971 France 71.19398 [52]US. Cl. 148/175; 23/232; 29/575; 117/106 R; 134/22 R; 134/22 C; 134/28;134/30; 137/15;148/186;148/189; 156/17 [51] Int. Cl. 110117/00; C23g1/08; B08b 9/06 [58] Field of Search 117/106 A, 106 C, 106 R;l48/174,1.6, 175; 134/2, 21, 22 R, 37, 22 C, 134/28, 30; 23/232 R;75/224; 156/17; 29/575; 137/15 [56] References Cited UNITED STATESPATENTS 2,935,429 5/1960 Grotlisch 134/28 2,948,642 8/1960 MacDonald148/l.5 2,965,523 12/1960 Englc [34/28 3,168,422 2/1965 Allegretti148/175 3,461,003 8/1969 Jackson 148/175 3,511,727 5/1970 Hays 156/17GAS USE 3,645,812 2/1972 Sussmann 148/175 OTHER PUBLICATIONS J. F.Pudvin and F. .I. Biondi, Transistor Technology, Vol. 111, D. VanNostrand Company, Inc., New York, N.Y., 1958, pages 102-116.

Primary Examiner-Morris O. Wolk Assistant Examiner-Dale LovercheckAttorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT Method fortransferring very pure ultimate use gas from a cylinder to a userapparatus through a tube fitted with valves and other distributing andcontrol elements, the tube being fitted, more particularly stated, forregulating flow and pressure. The tubing, valves and other elements aresubjected, before the transferring, and before assembly as a flow line,to chemical etching by aqueous solutions of volatile noncontaminatingacids, then to decontamination by extensive sweeping by a very pure hotgas to volatilize the acids. The ultimate use gas is subsequentlytransferred, after assembly and additional sweeping by a pure hot gas,through the decontaminated tubing, valves and other elements, to theultimate user apparatus, as for example in the transferring ofdopantcontaining gas in the manufacturing of semiconductors by epitaxyor diffusion.

9 Claims, 1 Drawing Figure GAS SUPPLY SOURCE METHOD FOR THE TRANSFER OFA GAS OF HIGH PURITY The present invention concerns a method and adevice for transferring very pure gas from a supply to a user apparatusthrough a pipe fitted with valves and other distributing and controlelements, more particularly for regulating flow and pressure. It appliesmore particularly to the transferring of gas intended for providing thedepositing of doping impurities at a predetermined low percentage onpellets made of a basic semiconductor material, for example, silicon,germanium, gallium arsenide, silicon carbide. etc., with a view tomanufacturing semiconductors, in installations for treating by epitaxyor by diffusion. or else for any other operation for the treatment ofsemiconductors requiring the presence of very pure gas. It is alsosuitable for other applications, such as the transferring of carrier gasfor analysis by chromatography or the trans ferring of gas for medicaluses.

The very pure gases supplied in industry are generally stored either ina cylinder under very high pressure, or sometimes. in the liquid statein a heat-proof container. Their impurity content is in the order of lvolume per million (10*). After possible vaporization and expansion,they are often subjected to additional purifying in a purifyingapparatus, for example, in the case of hydrogen, to a treatment byselective diffusion through a palladium or palladium alloy membrane.which further reduces their impurity contents.

Nevertheless, it has been observed that known transferring methods anddevices do not enable this high purity to be maintained. In the case ofthe manufacturing of semiconductors. the length of the life of minoritycarriers in the basic substance, which is originally in the order of1000 microseconds, can fall to a value in the order of 2 to 5microseconds after manufacturing of a junction'by epitaxy or diffusion.A treatment for trapping the impurities enables it to be raised again,but it generally remains less than 100 microseconds.

The aim of the present invention is to overcome the above disadvantages.and to provide a method and a device for transferring very pure gaswhich ensure the maintaining of that purity up to the using apparatus,

and which ensure, more particularly, in the case of gas intended .forthe manufacturing of semiconductors, after treatment of the basicsubstance by epitaxy by means of a very pure carrier gas charged with adosed quantity of the impurity chosen, the producing of semiconductorshaving a length oflife of the minority carriers which is substantiallygreater than 100 microsec onds.

The method according to the invention is characterized in that thetubing, valves and other distributing and control elements, aresubjected, prior to the transferring of gas. to chemical etching bymeans of acids which are not liable to introduce harmful impurities intothe gas, then to a decontamination treatment by extensive sweeping bymeans of a very pure gas.

It comprises, moreover, preferably at least one of the followingcharacteristics.

For the transferring of gas intended for depositing very smallquantities of a doping impurity in a pellet made of basic material witha view to manufacturing semiconductors, chemical etching is effected bymeans of acids free from doping impurities, preferably by means of amixture of nitric. hydrochloric and hydrofluoric acids having electronicpurity.

Each tubing element, each valve and each other element. are subjected,after etching, to extensive sweeping by means of a very pure hot gas;the elements are assembled in a place sheltered from impurities; then'the assembly is subjected to sweeping by means of a very pure gas.

The device according to the invention is characterized in that thevalves it comprises are manually or pneumatically operated. and in thatthe tubing, the valves and the other distributing and control elementshave their internal surfaces made of stainless steel, metal or alloy,silica glass, polymonochlorotrifluorethylene, or any other polymerhaving analogous mechanical strength, chemical inertia characteristicsand favorable characteristics under vacuum conditions, are etched bymeans of acids free from impurities liable to be drawn in with the gasto be transferred, subjected to a decontamination treatment by extensivesweeping by means of a very pure hot gas. then assembled. after thefluid-tight sealing, under vacuum conditions. of each component has beenascertained. 1n the case of the use of stainless steel tubing or valves,there are preferably made of steel containing approximately 18 percentchromium and 12 percent nickel.

It has been observed, more particularly, that the use of electromagneticvalves was to be abandoned. for they have certain disadvantages such asvibrations or overheating, and that the use of any elements comprisingglass other than silica glass, as well as the use of grease in vacuumconditions was also to be abandoned.

1n the case of the transferring of carrier gases for manufacturingsemiconductors by epitaxy or by diffusion, the use of any phosphoricacid. which would be liable to contaminate the gas. for the etchingoperation is more particularly to be avoided, and nitric hydrochloricand hydrofluoric acids having what is known as electronic purity, suchas they are used for treating silicon, which is the usual basic materialin production, should be used.

The assembling of a device for the transferring of very pure hydrogenfrom a storage cylinder under high pressure to a treatment furnace fordepositing by epitaxy is described herebelow by way of an example withreference to the accompanying FIGURE which shows a schematicrepresentation of an embodiment of gas supply connection tubing and useapparatus according to the present invention.

The elements of the gas transferring device from a gas supplying source1 to a gas use apparatus 2 include tubing 4, connections 5, valves 3,flowmeters 6 which comprise, for example at least on their internalsurface, stainless steel containing approximately 18 percent chromiumand 12 percent nickel, such as that corresponding to AFNOR standard Z3CND 18-12 or to designation 316L of US standard AlSl, are subjected,after removal of grease with a hot chlorated solvent such astrichlorethylene, to chemical etching in a bath at 60C having avolumetric composition of about 16 percent nitric acid at adensity of1.42, 3 percent hydrochloric acid at a density of 1.189, 4 percent byvolume of hydrofluoric acid at a density of 1.18 and 77 percentde-ionized water, for several minutes. then they are washed, rinsed anddried. They are then passivated by immersion in a cold nitric acid bathat a density of 1.1 15 during about 15 minutes, rinsed then subjected toa decontamination treatment by sweeping by means of a very pure hot gas.for example nitrogen at 150C. for about an hour.

All the above operations must be performed in a fume cupboard of thekind known as controlled laminar flow type. and in usual conditions forthe treating of the semiconductor substance itself.

The various elements are then stored in filtered air and assembled.after the fluid-tight sealing of each of them under vacuum conditionshas been ascertained. After the fluid-tight sealing of the assemblyunder vacuum conditions has been ascertained. the latter is subjected toa decontamination treatment by sweeping by means of a very pure gas. forexample. hydrogen containing not more than 0.1 volume per million ofoxygen and dampness. during several days.

Although the method and device which have just been described are to beconsidered as preferable. it will be understood that variousmodifications may be made thereto without going beyond the scope of theinvention. it being possible to replace certain operations of the methodby others which would have the same technical function therein. and toreplace certain substances of the device by others having equivalentproperties.

We claim:

I. A method for transferring very pure use gas from a supply to a userapparatus through tubing fitted with valves and other distributing andcontrol elements for regulating the flow and pressure of said use gas,said method comprising:

chemically etching the tubing. valves and other distributing and controlelements prior to assembly with an aqueous solution of volatile acidswhich are free from impurities which might be introduced into the usegas:

decontaminating the chemically etched tubing.

valves and other distributing and control elements so formed byextensively sweeping them with a very pure sweeping gas:

assembling said tubing. valves and other distributing and controlelements together; and passing said very pure use gas from said supplyto said user apparatus through the thus etched and decontaminatedtubing.

2. The method according to claim I, wherein said tubing. valves andother distributing and control elements are further decontaminated afterassembly but prior to the passage of said very pure use gas therethroughby sweeping with another very pure sweeping gas.

3. The method according to claim 1, wherein said another very puresweeping gas is hydrogen containing not more than about 0.l volume permillion of oxygen and water vapor. and further wherein said very puresweeping gas is nitrogen.

4. The method according to claim 1, wherein said volatile acid isselected from the group consisting of nitric acid, hydrochloric acid andhydrofluoric acid.

5. A method according to claim 1, wherein said tubing. valves and otherdistributing and control elements are washed. rinsed and dried afterchemical etching and prior to sweeping with decontaminating gas.

6. A method according to claim 5, wherein said tubing. valves and otherdistributing and control elements are passivated by immersion in a coldnitric acid bath after washing. rinsing and drying and prior to sweepingwith decontaminating gas.

7. In a process for making a semiconductor in which a use gas consistingessentially ofa very pure carrier gas charged with a doping quantity ofa doping impurity is transferred from a use gas supply to a userapparatus where said doping impurity is deposited on a basicsemiconductor material at a predetermined low percentage. said use gasbeing transferred from said use gas supply to said user apparatus viatubing fitted with valves and other distributing and control elementsfor regulating the flow and pressure of said use gas. the improvementfor maintaining the purity of said very pure use gas comprisingchemically etching the tubing. valves and other distributing and controlelements prior to assembly together in a bath of an aqueous volatileacid selected from the group consisting of nitric acid. hydrochloricacid and hydrofluoric acid and mixtures thereof; decontaminating thechemically etched tubing. valves and other distributing and controlelements by extensively sweeping them with a first very puredecontaminating gas; assembling said tubing. valves and otherdistributing and control elements together". and sweeping said tubing.valves and other distributing and control elements with a second verypure decontaminating gas prior to the transfer of said use gastherethrough.

8. The method according to claim 7, further comprising passivating thetubing. valves and other distributing and control elements in nitricacid after the chemical etch but prior to the sweeping withdecontaminating gas with the first very pure decontaminating gas.

9. The method according to claim 8, further comprising washing, rinsingand drying the tubing. valves and other distributing and controlelements after the chemical etch and prior to passivation.

1. A METHOD FOR TRANSFERRING VERY PURE USE GAS FROM A SUPPLY TO A USERAPPARATUS THROUGH TUBING FITTED WITH VALVES AND OTHER DISTRIBUTING ANDCONTROL ELEMENTS FOR REGULATING THE FLOW AND PRESSURE OF SAID USE GAS,SAID METHOD COMPRISING: CHEMICALLY ETCHING THE TUBING, VALVES AND OTHERDISTRIBUTING AND CONTROL ELEMENTS PRIOR TO ASSEMBLY WITH AN AQUEOUSSOLUTION OF VOLATILE ACIDS WHICH ARE FREE FROM IMPURITIES WHICH MIGHT BEINTRODUCED INTO THE USE GAS, DECONTAMINATING THE CHEMICALLY ETCHEDTUBING, VALVES AND OTHER DISTRIBUTING AND CONTROL ELEMENTS SO FORMED BYEXTENSIVELY SWEEPING THEM WITH A VERY PURE SWEEPING GAS, ASSEMBLING SAIDTUBING, VALVES AND OTHER DISTRIBUTING AND CONTROL ELEMENTS TOGETHER, ANDPASSING SAID VERY PURE USE GAS FROM SAID SUPPLY TO SAID USER APPARATUSTHROUGH THE THUS ETCHED AND DECONTAMINATED TUBING.
 2. The methodaccording to claim 1, wherein said tubing, valves and other distributingand control elements are further decontaminated after assembly but priorto the passage of said very pure use gas therethrough by sweeping withanother very pure sweeping gas.
 3. The method according to claim 1,wherein said another very pure sweeping gas is hydrogen containing notmore than about 0.1 volume per million of oxygen and water vapor, andfurther wherein said very pure sweeping gas is nitrogen.
 4. The methodaccording to claim 1, wherein said volatile acid is selected from thegroup consisting of nitric acid, hydrochloric acid and hydrofluoricacid.
 5. A method according to claim 1, wherein said tubing, valves andother distributing and control elements are washed, rinsed and driedafter chemical etching and prior to sweeping with decontaminating gas.6. A method according to claim 5, wherein said tubing, valves and otherdistributing and control elements are passivated by immersion in a coldnitric acid baTh after washing, rinsing and drying and prior to sweepingwith decontaminating gas.
 7. In a process for making a semiconductor inwhich a use gas consisting essentially of a very pure carrier gascharged with a doping quantity of a doping impurity is transferred froma use gas supply to a user apparatus where said doping impurity isdeposited on a basic semiconductor material at a predetermined lowpercentage, said use gas being transferred from said use gas supply tosaid user apparatus via tubing fitted with valves and other distributingand control elements for regulating the flow and pressure of said usegas, the improvement for maintaining the purity of said very pure usegas comprising chemically etching the tubing, valves and otherdistributing and control elements prior to assembly together in a bathof an aqueous volatile acid selected from the group consisting of nitricacid, hydrochloric acid and hydrofluoric acid and mixtures thereof;decontaminating the chemically etched tubing, valves and otherdistributing and control elements by extensively sweeping them with afirst very pure decontaminating gas; assembling said tubing, valves andother distributing and control elements together; and sweeping saidtubing, valves and other distributing and control elements with a secondvery pure decontaminating gas prior to the transfer of said use gastherethrough.
 8. The method according to claim 7, further comprisingpassivating the tubing, valves and other distributing and controlelements in nitric acid after the chemical etch but prior to thesweeping with decontaminating gas with the first very puredecontaminating gas.
 9. The method according to claim 8, furthercomprising washing, rinsing and drying the tubing, valves and otherdistributing and control elements after the chemical etch and prior topassivation.